'\" et
.TH SEND "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
send
\(em send a message on a socket
.SH SYNOPSIS
.LP
.nf
#include <sys/socket.h>
.P
ssize_t send(int \fIsocket\fP, const void *\fIbuffer\fP, size_t \fIlength\fP, int \fIflags\fP);
.fi
.SH DESCRIPTION
The
\fIsend\fR()
function shall initiate transmission of a message from the specified
socket to its peer. The
\fIsend\fR()
function shall send a message only when the socket is connected. If
the socket is a connectionless-mode socket, the message shall be sent
to the pre-specified peer address.
.P
The
\fIsend\fR()
function takes the following arguments:
.IP "\fIsocket\fR" 12
Specifies the socket file descriptor.
.IP "\fIbuffer\fR" 12
Points to the buffer containing the message to send.
.IP "\fIlength\fR" 12
Specifies the length of the message in bytes.
.IP "\fIflags\fR" 12
Specifies the type of message transmission. Values of this argument are
formed by logically OR'ing zero or more of the following flags:
.RS 12 
.IP MSG_EOR 14
Terminates a record (if supported by the protocol).
.IP MSG_OOB 14
Sends out-of-band data on sockets that support out-of-band
communications. The significance and semantics of out-of-band data are
protocol-specific.
.IP MSG_NOSIGNAL 14
Requests not to send the SIGPIPE signal if an attempt to send is made
on a stream-oriented socket that is no longer connected. The
.BR [EPIPE] 
error shall still be returned.
.RE
.P
The length of the message to be sent is specified by the
.IR length
argument. If the message is too long to pass through the underlying
protocol,
\fIsend\fR()
shall fail and no data shall be transmitted.
.P
Successful completion of a call to
\fIsend\fR()
does not guarantee delivery of the message. A return value of \-1
indicates only locally-detected errors.
.P
If space is not available at the sending socket to hold the message to
be transmitted, and the socket file descriptor does not have O_NONBLOCK
set,
\fIsend\fR()
shall block until space is available. If space is not available at the
sending socket to hold the message to be transmitted, and the socket
file descriptor does have O_NONBLOCK set,
\fIsend\fR()
shall fail. The
\fIselect\fR()
and
\fIpoll\fR()
functions can be used to determine when it is possible to send more
data.
.P
The socket in use may require the process to have appropriate
privileges to use the
\fIsend\fR()
function.
.SH "RETURN VALUE"
Upon successful completion,
\fIsend\fR()
shall return the number of bytes sent. Otherwise, \-1 shall be
returned and
.IR errno
set to indicate the error.
.SH ERRORS
The
\fIsend\fR()
function shall fail if:
.TP
.BR EAGAIN " or " EWOULDBLOCK
.br
The socket's file descriptor is marked O_NONBLOCK and the requested
operation would block.
.TP
.BR EBADF
The
.IR socket
argument is not a valid file descriptor.
.TP
.BR ECONNRESET
A connection was forcibly closed by a peer.
.TP
.BR EDESTADDRREQ
.br
The socket is not connection-mode and no peer address is set.
.TP
.BR EINTR
A signal interrupted
\fIsend\fR()
before any data was transmitted.
.TP
.BR EMSGSIZE
The message is too large to be sent all at once, as the socket requires.
.TP
.BR ENOTCONN
The socket is not connected.
.TP
.BR ENOTSOCK
The
.IR socket
argument does not refer to a socket.
.TP
.BR EOPNOTSUPP
The
.IR socket
argument is associated with a socket that does not support one or more
of the values set in
.IR flags .
.TP
.BR EPIPE
The socket is shut down for writing, or the socket is connection-mode
and is no longer connected. In the latter case, and if the socket is of
type SOCK_STREAM or SOCK_SEQPACKET and the MSG_NOSIGNAL flag is not set,
the SIGPIPE signal is generated to the calling thread.
.P
The
\fIsend\fR()
function may fail if:
.TP
.BR EACCES
The calling process does not have appropriate privileges.
.TP
.BR EIO
An I/O error occurred while reading from or writing to the file system.
.TP
.BR ENETDOWN
The local network interface used to reach the destination is down.
.TP
.BR ENETUNREACH
.br
No route to the network is present.
.TP
.BR ENOBUFS
Insufficient resources were available in the system to perform the
operation.
.LP
.IR "The following sections are informative."
.SH "EXAMPLES"
None.
.SH "APPLICATION USAGE"
If the
.IR socket
argument refers to a connection-mode socket, the
\fIsend\fR()
function is equivalent to
\fIsendto\fR()
(with any value for the
.IR dest_addr
and
.IR dest_len
arguments, as they are ignored in this case). If the
.IR socket
argument refers to a socket and the
.IR flags
argument is 0, the
\fIsend\fR()
function is equivalent to
\fIwrite\fR().
.SH "RATIONALE"
None.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "\fIconnect\fR\^(\|)",
.IR "\fIgetsockopt\fR\^(\|)",
.IR "\fIpoll\fR\^(\|)",
.IR "\fIpselect\fR\^(\|)",
.IR "\fIrecv\fR\^(\|)",
.IR "\fIrecvfrom\fR\^(\|)",
.IR "\fIrecvmsg\fR\^(\|)",
.IR "\fIsendmsg\fR\^(\|)",
.IR "\fIsendto\fR\^(\|)",
.IR "\fIsetsockopt\fR\^(\|)",
.IR "\fIshutdown\fR\^(\|)",
.IR "\fIsocket\fR\^(\|)",
.IR "\fIwrite\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<sys_socket.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .